In order to verify the possibility of submarine hot spring discharges along the coast of Beppu Bay, we used a radon (Rn) towing survey method using a small vessel, which has been frequently used in recent years to survey coastal submarine groundwater discharge (SGD). Analysis of the data using the "Rn concentration and salinity mixing diagram," which includes data from hot springs, groundwater (springs and well water), and river water in the land area along the coast, showed that there were four areas with a high mixing ratio of groundwater. As a result of investigating the relationship between these four locations and estimated hot spring flow paths and subsurface structure of the terrestrial area, and the relation between radon concentration and water temperature in these four locations, it was shown that two of the locations may be submarine hot spring discharge areas.

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The five lagoons along the coastline of the Tokachi region, Hokkadio, sporadically open by breaking each sandbar from the overflow of lagoon water at the lowest part of the sandbar. In order to explore formation processes of the lagoons, sediment cores ca. 10 m long were sampled on the sandbars of Oikamanai Lagoon and Yudo Lagoon, and the C14 dating for the cores was performed. The origin of gravels in the cores was then identified by specifying the rock species. Meanwhile, the hydrological function of the sandbars was investigated by obtaining the hydraulic conductivity of the cores by permeability tests and grain size analyses. All the analyses for the cores gave the following findings: 1) The gravels in the cores were transported into the sea by fluvial erosion of the Hidaka metamorphic zone, deposited by flood tides or tsunamis, 2) the prototype of Oikamanai Lagoon was produced ca. 6,000 yrs ago followed by the gravel layer formation ca. 5,400 – 4,800 yrs ago and the buildup of the nearly present sandbar by ca. 4,800 yr ago, and 3) under closed condition, the lagoon water is likely to outflow into the sea as confined groundwater through the pebble layer in sandbar.

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